Silicon-germanium solar cell having a high power efficiency

ABSTRACT

A silicon-germanium thin-film solar cell having a quantum well structure as an active base layer within the space-charge region of the silicon p-n diode junction. The quantum well structure is composed of a sequence of silicon and germanium layers. In this manner, a highly absorbent base layer is produced in a silicon solar cell.

[0001] The present application claims priority from German Application No. 10061192.3, filed Dec. 8, 2000, and hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] The present invention is directed to a silicon-germanium thin-film solar cell, as well as to a method for making such a solar cell.

[0003] The power efficiency of thin-film solar cells is determined by their absorptivity. However, silicon solar cells, whose film thickness has been reduced in the active regions for reasons of economics, also experience a reduction in the short-circuit current. To compensate for this, in turn, it is necessary to employ film systems having a highest possible absorptivity in the active region of the cell. Germanium, a material having a smaller band energy gap than silicon, has shown great promise for such a use. For quite some time, silicon-germanium compounds (SiGe) have been known for their outstanding properties, particularly with respect to their compatibility to pure silicon semiconductor technology.

[0004] The publication by Said et al. (Design, Fabrication, and Analysis of Crystalline Si-SiGe Heterostructure Thin-Film Solar Cells, IEEE Trans. on Electr. Dev., vol. 46, no. 10, p. 2103 (1999)), describes a solar cell that is constructed of Si/SiGe heterostructure films. A sharp increase in the power efficiency of the cell is expected from the use of hetero-films. However, increased germanium concentrations caused by the lattice mismatch of the germanium with respect to the silicon substrate, make it necessary to overcome the difficulty of epitaxial deposition of the layers. The dislocations resulting from the mismatching prove to be especially troublesome. The increasing dislocation density due, in particular, to so-called threading dislocations running at the surface, in the absorption region (p-n space-charge region) of the cell, causes the dark current to rise sharply; this leads directly to a decrease in the open terminal voltage (Voc) and to a poor fullness factor of the cell. Just as undesirable is the relaxation of grid bias, which has a direct, negative effect on the band structure and, thus, also on the power efficiency.

[0005] A special germanium film system is described in the publication by Brunhes et al. (Electroluminescence of Ge/Si Self-Assembled Quantum Dots Grown by Chemical Vapor Deposition, Appl. Phys. Lett., vol. 77, no. 12, p. 1822 (2000)). To manufacture light-emitting diodes, germanium is deposited in island patterns on the silicon substrate. The Ge islands raise the electro- and photoluminescence due to the resonance in the wavelength range of between 1.4-1.5 micrometers.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to improve the power efficiency of a silicon-germanium solar cell.

[0007] The present invention provides a silicon-germanium thin-film solar cell, wherein a quantum well structure composed of a sequence of silicon and germanium layers is arranged on a silicon substrate (1, 2, 3), within the space-charge region of the silicon p-n diode junction (6, 7).

[0008] The layer sequence may be constructed of a quantum well layer (41), a wetting layer (42), a Ge island layer (43), and of an Si cover layer (5), in multiple succession, which exhibits a quantum well structure having a smaller band gap than silicon. The layer sequence may constructed of a first quantum well layer (44), of a Ge layer (45), of a first Si layer (46), of a second Ge layer (47), of a second quantum well layer (48), and of a Si cover layer (5), in multiple succession, which exhibits a quantum well structure having a smaller band gap than silicon. The layer sequence may constructed of a multiple first sequence of a Ge layer (49), an Si layer (410), and of a multiple second sequence of the first sequence, and of an Si cover layer (5), which exhibits a quantum well structure having a smaller band gap than silicon. The sequence may include a 10 to 20-times repetition of the particular layers.

[0009] Also provided is a method, wherein the layer sequence is produced using molecular-beam epitaxy (MBE), low-pressure vapor-phase epitaxy (LP-CVD), or ultra high-vacuum vapor-phase epitaxy (UHV-CVD).

[0010] The present invention includes a silicon (Si)/silicon-germanium (Si/Ge) thin-film solar cell having a quantum well structure within the space-charge region of the silicon p-n diode junction. The quantum well structure is composed of a sequence of silicon and germanium layers. In this manner, a highly absorbent base layer is produced in a silicon-based solar cell.

[0011] On the one hand, the layer sequence of the quantum well structure is formed by the epitaxial growth of self-organizing germanium islands and/or by the deposition of a few atomic monolayers (ML) of silicon and germanium. On the other hand, it is formed from epitaxial Si_(n)Ge_(m) ultrathin superlattices (m ML Si; n ML Ge; 1 ML=0.14 nm) on a p-doped silicon substrate. The epitaxial methods used include molecular-beam epitaxy (MBE), low-pressure (LP-CVD) or ultra high-vacuum vapor-phase epitaxy (UHV-CVD).

[0012] A significant advantage of the present invention resides in the improved power efficiency in comparison to conventional silicon solar cells, due to a substantially enhanced absorptivity in the base, in particular within the longer-wave region (λ>λ_(g) ^(S1)) of solar radiation. The above-mentioned, epitaxial deposition of SiGe quantum wells (QWs) and Ge islands, which grow in self-organized fashion in the base without the formation of mismatch dislocations at the Si/SiGe interface, causes the short-circuit current of the cell to rise, without the terminal voltage noticeably subsiding in comparison to the Si reference cell. The Si/SiGe layer sequence of the present invention, which is deposited on the Si substrate and is elastically strained (pseudomorphic), has a band gap which is smaller than that of Si and which absorbs the longer-wave photons below the Si band gap. Therefore, given an unvarying, active layer thickness, the short-circuit current of the cell rises sharply without any attendant, substantial increase in the dark current.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention is elucidated in the following in light of advantageous exemplary embodiments, reference being made to schematic figures in the drawing, which show:

[0014]FIG. 1 a schematic layer construction of the Ge island solar cell;

[0015]FIG. 2 a characteristic band pattern of the Ge island solar cell;

[0016]FIG. 3 a schematic layer construction of the solar cell including Ge and Si quantum well layers;

[0017]FIG. 4 a schematic layer construction of the solar cell including Si_(n)Ge_(m) ultrathin superlattices.

DETAILED DESCRIPTION

[0018] A first exemplary embodiment in accordance with FIG. 1 shows the layer construction of the Ge island solar cell on a silicon <100> substrate: (8) Si, n⁺⁺>5· 10¹⁹ cm⁻³ 10 nm Emitter (7) Si, n⁺5·10¹⁸ cm³ 20 nm layers (6) Si, p⁻2 10¹⁵ cm⁻³ 750 nm Interface Base (5) Si, p⁻2·10¹⁵ cm⁻³ 15 nm Si intermediate layer layers approx. (43) Ge, p⁻2·10¹⁵ cm⁻³ 8 ML germanium islands 10 X repetition (42) Ge, p⁻2·10¹⁵ cm⁻³ 4 ML wetting layer of the base (41) SiGe_(0.3) p⁻2·10¹⁵ cm⁻³ 16 ML quantum well layer layers (3) Si, p⁻2·10¹⁵ cm⁻³ 100 nm silicon <100>, (2) Si,p bulk ρ=10 Ωcm rear side contact a few (implanted or Substrate (1) Si, p⁺⁺1·10¹⁹ −5·10²⁰ cm⁻³ nm diffused)

[0019] As a highly absorbent base layer, the active region is composed of two parts: a wider two-dimensional Si_(1−x)Ge_(x)QW layer 41 having a relatively low Ge concentration, for example 16 ML=2.2 nm SiGe_(0.3), and a thinner layer having a high Ge concentration 42, for example 4 ML Ge, which is used as a wetting layer. This part is alternatively composed of two Si_(1−x)Ge_(x) QW alloying layers (or Ge_(m)/Si₂₀/Ge_(m), n=2.4) having different Ge concentrations, or of two Si_(m)Ge_(n) superlattices having a different, effective Ge concentration.

[0020] The second part is composed of three-dimensional Ge islands 43, which nucleate on the two-dimensionally grown Ge wetting layer when appropriate deposition parameters (MBE at approx. T=700 C) are selected, without producing any mismatch dislocations at the subjacent hetero-interfaces (Si/SiGe_(0.3) or SiGe_(0.3)/Ge). The three-dimensional Ge islands 43 grow on wetting layer 42 in accordance with the so-called Stranski-Krastanov model, in which epitaxial island growth takes place in response to the energy conditions prevailing at the layer surface of the lattice-mismatched semiconductor layer systems. Temperature is an important parameter in layer fabrication, as it controls the diffusion mechanisms on the solid surface. The deposition preferably takes place within a temperature range extending from 500 to 700 C. Disposed between the weakly p-doped base layers (2 10¹⁵cm⁻³; see FIGS. 1, 3, 4) and the n-doped emitter layers is a 500-750 nm thick, p-doped Si interface, which spatially separates the p-n junction from the Si/SiGe hetero-junction, and also places the active layers at an optimal distance to the emitter surface (see K. Said et al., IEEE EDL 46, p. 2103 (1999)). The dopant levels are selected in such a way that the SiGe quantum wells are situated within the space-charge region.

[0021] By incorporating these layers in the emitter space-charge region of the outer Si cell, as described above, one produces the characteristic band pattern sketched in FIG. 2. The holes photogenerated and localized in the Ge/SiGe and/or SiGe double wells are advanced by the strong electric field (intense band bending) in the space-charge region, from the localized state of the less deep Ge/SiGe quantum well, due to thermal emission or tunneling from the localized state, into the Si valence band, and, as a result of the built-in voltage, to the outer Si contacts, and, thus, contribute to the photo current, thereby significantly enhancing the power efficiency of the cell as compared to the Si reference cell.

[0022] A further exemplary embodiment in accordance with FIG. 3 shows the layer construction of the solar cell having Ge and Si quantum well layers on a silicon <100> substrate: (8) Si, n⁺⁺ 10 nm Emitter layers (7) Si, n⁺ 20 nm (6) Si,p⁻1 10¹⁶ cm⁻³ 500 nm interface (5) Si, p⁻ 25 nm (48) SiGe_(0.25), p⁻1·1O¹⁷ cm⁻³ 10 nm (47) Ge, p 1· 10 ¹⁷ cm⁻³ 4 ML Base layers (46) Si, p 1· 10 ¹⁷ cm⁻³ 20 ML approx. 10X repetition of the (45) Ge, p 1· 10 ¹⁷ cm⁻³ 4 ML base layers (44) SiGe_(0.15-0.2) , p−1·10¹⁶ cm⁻³ 10 nm (3)Si, p−1·10¹⁶ cm−3 100 nm silicon <100>, (2)Si,p bulk ρ=10 Ωcm a few Substrate (1) Si, p⁺⁺1·10¹⁹ −5·10²⁰ cm⁻³ nm

[0023] A further exemplary embodiment in accordance with FIG. 4 shows the layer construction of the solar cell having Si_(n)Ge_(m) ultrathin superlattices on a silicon <100> substrate: (8) Si, n⁺⁺ 10 nm Emitter (7) Si, n⁺ 20 nm layers (6) Si, p⁻2 10¹⁵ cm⁻³ 750 nm interface (5) Si, p⁻1 10¹⁵ cm⁻³ 10 nm Base (413) Si, p 2·10¹⁵ cm⁻³ 5 ML Approx. 8X layers repetition of the Si approx. 20 and Ge layers up to X approx. 9 nm repetition (412) Ge, p⁻2·10¹⁵ cm⁻³ 5 ML thickness of the base layers (411) Si, p 2 10¹⁵ cm⁻³ 10 nm (410) Si, p 2 10¹⁵ cm⁻³ 7 ML (49) Ge, p⁻2·10¹⁵ cm⁻³ 3 ML (3) Si, p⁻2·10¹⁵ cm⁻³ 100 nm silicon <100>, (2) Si,p bulk ρ=10 Ωcm a few Substrate (1) Si, p⁺⁺1·10¹⁹ −5·10²⁰ cm⁻³ nm 

What is claimed is:
 1. A silicon-germanium thin-film solar cell comprising: a silicon substrate; and a quantum well structure composed of a layer sequence of silicon and germanium layers arranged on the silicon substrate within a space-charge region of a silicon p-n diode junction.
 2. The solar cell as recited in claim 1, wherein the layer sequence includes a quantum well layer, a wetting layer, a Ge island layer, and an Si cover layer, in multiple succession, so as to define a quantum well structure having a smaller band gap than silicon.
 3. The solar cell as recited in claim 1, wherein the layer sequence includes a first quantum well layer, a Ge layer, a first Si layer, a second Ge layer, a second quantum well layer, and a Si cover layer, in multiple succession, so as to define a quantum well structure having a smaller band gap than silicon.
 4. The solar cell as recited in claim 1, wherein the layer sequence includes a multiple first sequence of a Ge layer and an Si layer, and a multiple second sequence of the first sequence and an Si cover layer, so as to define a quantum well structure having, a smaller band gap than silicon.
 5. The solar cell as recited in claim 2 wherein the layer sequence is repeated 10 to 20-times.
 6. The solar cell as recited in claim 3 wherein the layer sequence is repeated 10 to 20-times.
 7. The solar cell as recited in claim 4 wherein the layer sequence is repeated 10 to 20-times.
 8. A method for producing a silicon-germanium thin-film solar cell comprising a silicon substrate and a quantum well structure composed of a layer sequence of silicon and germanium layers arranged on the silicon substrate within a space-charge region of a silicon p-n diode junction, the method comprising the step of: producing the layer sequence using molecular-beam epitaxy (MBE), low-pressure vapor-phase epitaxy (LP-CVD), or ultra high-vacuum vapor-phase epitaxy (UHV-CVD).
 9. The method as recited in claim 8, wherein the layer sequence includes a quantum well layer, a wetting layer, a Ge island layer, and an Si cover layer, in multiple succession, so as to define a quantum well structure having a smaller band gap than silicon.
 10. The method as recited in claim 8, wherein the layer sequence includes a first quantum well layer, a Ge layer, a first Si layer, a second Ge layer, a second quantum well layer, and a Si cover layer, in multiple succession, so as to define a quantum well structure having a smaller band gap than silicon.
 11. The method as recited in claim 8, wherein the layer sequence includes a multiple first sequence of a Ge layer and an Si layer, and a multiple second sequence of the first sequence and an Si cover layer, so as to define a quantum well structure having a smaller band gap than silicon.
 12. The method as recited in claim 9, wherein the layer sequence is repeated 10 to 20-times.
 13. The method as recited in claim 10, wherein the layer sequence is repeated 10 to 20-times.
 14. The method as recited in claim 11, wherein the layer sequence is repeated 10 to 20-times. 